Liquid crystal optical element and test method for its boundary layer

ABSTRACT

On substrates  2   a   , 2   b  of a chiral nematic liquid crystal optical element  1 , transparent electrodes  3   a   , 3   b  and electrical insulation layers  4   a   , 4   b  are formed, and further resin layers  5   a   , 5   b  having a pencil hardness of “B” or less are formed on the electrical insulation layers by a spin coating method so as to be in contact with a liquid crystal layer  7 . When the surface hardness of the resin layers is to be measured, a glass substrate on which a resin layer is formed by screen-printing is prepared as a test piece, and the test piece is fitted to a pencil-scratching tester. The surface hardness is measured by scratching the test piece with two kinds of testing pencil selected from testing pencils having 17 grades of density.

CROSS-REFERENCE TO RELATED APPLICATIONS

The entire disclosure of Japanese Patent Application No. 2000-402045filed on Dec. 28, 2000, including specification, claims, drawings andsummary are incorporated herein by reference in their entirely.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal optical elementprovided with liquid crystal which exhibits two or more optically stablestates in a non-voltage-application time.

2. Description of the Background

A chiral nematic liquid crystal optical element (hereinbelow, referredto as CL-LCD) has a phase-change type mode. It provides selectivereflection in a planar state (hereinbelow, referred to as PL) andprovides a scattering state in a focalconic state (hereinbelow, referredto as FC). By applying a predetermined voltage across the electrodes,the liquid crystal can be transformed into PL or FC. For example, whenit is transformed from FC to PL, the liquid crystal is once rendered tobe a homeotropic state (a state that liquid crystal molecules arealigned vertically to the substrate plane, and hereinbelow, referred toas HO), and then, it is transformed into PL. Then, the liquid crystal isstable in PL or FC even in a non-voltage-application time, and eitherstate can be maintained.

Description will be made as to an optical display state. In FC, aslightly scattering state of incident light is produced, and in PL, aselective reflection of incident light is produced. Further, byadjusting the helical pitch (λ_(AVG)) of the liquid crystal layer, anoperation mode of “transmittance-scattering” or a color displayutilizing colors of selectively reflected light can be effected.

Electrodes for CL-LCD, and a display state of it and so on are describedalready in U.S. Ser. No. 09/813,988 (preliminary application No., filedon Mar. 22, 2001), and the relation of a driving method and a displaystate thereof and so on are described already in U.S. Patent Applicationfiled on Apr. 2, 2001 by Makoto Nagai et al (no application number givenyet). The present application refers to and includes portions relatingto these applications.

Next, description will be made as to a unique problem of CL-LCDoriginated from a memory state. The state of CL-LCD is made to be amemory state in FC or PL, and it is left for a predetermined time in anon-voltage-application state. Then, even when a voltage correspondingto a new image is applied so as to change the display, a “image-stickingphenomenon” wherein the previous display remains takes place.

In the following, such image-sticking phenomenon will be explained bytaking a dot matrix type CL-LCD having 160 row electrodes and 160 columnelectrodes as an example. In this case, it is assumed that “an alignmentlayer” in contact with the liquid crystal layer at an inner side of thesubstrates is made of the same material as used for a TN or STN liquidcrystal display element. Generally, since a surface of an aligning layerin the TN or STN liquid crystal display element is subjected to analigning treatment by rubbing, a material having a high surface hardness(for example, 3H-6H in a pencil hardness test method) is used. In CL-LCDtoo for example, the same technique is used. As a concrete example on analigning layer in contact with the liquid crystal layer, there is knowna case of providing polyimide or a case without using any aligning layer(U.S. Pat. No. 5,453,863).

Then, a background portion of a display area of dot matrix type is madeto be FC, and portions corresponding to characters, figures and so onare made to be PL. An example of displaying a character of “A” is shownin FIG. 4( a). After such predetermined image has been provided, theapplication of a voltage is stopped, and the display panel is left for along time in a incubator of 60° C.

Then, a voltage which makes the liquid crystal in the whole display areato be HO is applied. When the liquid crystal in the whole area becomesHO, the display in the whole area is extinguished. When a voltage isapplied subsequently so that the liquid crystal in the whole areabecomes FC, the whole area does not provide an uniform color but thecharacter of “A” which has been previously displayed remains slightly,and the character is observed as shown in FIG. 4( b). Further, even in acase that the liquid crystal of the whole area is made to be PL via HOafter the panel has been left for a long time, the display of “A”remains.

On the contrary, the image-sticking phenomenon takes place even in thefollowing case. Namely, the liquid crystal in the background portion ismade to be PL, and the liquid crystal in the portions corresponding tocharacters and so on is made to be FC. The display panel in such statesis left for a long time, and then, a display in the whole area isextinguished by applying a voltage by which the state of the liquidcrystal in the whole area of display becomes HO (the liquid crystalitself is in a transparent state). Subsequently, the whole display areais changed to PL or FC. In particular, the image-sticking phenomenon isapt to be observed in a case that a display state wherein the liquidcrystal corresponding to the background portion is FC and the liquidcrystal corresponding to characters and so on is PL is left for a longtime, and then, the liquid crystal in the whole display area is made tobe FC.

Further, image-sticking takes place as well in a case of CL-LCD havingsegment type electrodes. For instance, the image-sticking occurs when adisplay state wherein a display portion is PL and a background portionis FC is maintained for a long time, and then, such display is changedto another display.

In FIG. 5( a), all 7 segments are ON to display a character of “8”. Acolor display in combination of black in a background portion in FC anda selective reflection in a visible range in a segment portion in PL, isprovided. In FIG. 5( b), when a character of “5” is newly displayed,upper right and lower left segment portions are lit weakly.

However, when a display is performed in combination of PL and FC inCL-LCD, a PL portion is generally light and the most high visibility isobtainable. Accordingly, in a case of the segment type display in FIG.5, a possibility of causing erroneous recognition is relatively lowerthan the case of the dot matrix type in FIG. 4.

The image-sticking phenomenon remains slightly even after a voltage forproviding HO has continuously been applied and a state that the entiredisplay area is erased has been continued for several hours. Further, adisplay state having a image-sticking phenomenon (hereinbelow, referredto as image-sticking) could not be completely erased even after anisotropic state which is caused by heating the liquid crystal has beencontinued for several hours.

Accordingly, it is an object of the present invention to provide CL-LCDcapable of preventing the occurrence of image-sticking even though theliquid crystal is left for a long time while the state of the liquidcrystal is constant. Further, the present invention is to provide aliquid crystal optical element having excellent function withoutchanging largely the conventional manufacturing method, and a testmethod for the liquid crystal optical element.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda liquid crystal optical element comprising a pair of substrates withelectrodes between which a liquid crystal layer including a chiralnematic liquid crystal is interposed wherein at least one of thesubstrates is transparent and the liquid crystal layer exhibits two ormore optically stable states in a non-voltage-application time, theliquid crystal optical element being characterized in that a boundarylayer is provided between an electrode on at least one of the substratesand the liquid crystal layer, and the surface hardness of the boundarylayer is B or less in terms of a pencil hardness test.

In a second aspect, the above-mentioned liquid crystal optical elementwherein the boundary layer is a resin layer is provided. In a thirdaspect, the above-mentioned liquid crystal optical element wherein theresin layer is of polyimide is provided. In a fourth aspects, theabove-mentioned liquid crystal optical element wherein the electrode onat least one of the substrates is divided into a plurality of portionsis provided. In a fifth aspect, the above-mentioned liquid crystaloptical element wherein the surface hardness is 3B or less is provided.In a sixth aspect, the above-mentioned liquid crystal optical elementwherein a dot matrix display is performed with the electrodes isprovided. In a seventh aspect, the above-mentioned liquid crystaloptical element wherein a segment display is performed with theelectrodes is provided. In an eighth aspect, the above-mentioned liquidcrystal optical element wherein the heat distortion temperature of amaterial for forming the boundary layer is 50° C. or more, and thecoefficient of elasticity at room temperature is 1 kPa or more, isprovided.

Further, according to a ninth aspect, there is provided a test methodfor a liquid crystal optical element comprising a pair of substrateswith electrodes between which a liquid crystal layer including a chiralnematic liquid crystal is interposed wherein at least one of thesubstrates is transparent and the liquid crystal layer exhibits two ormore optically stable states in a non-voltage-application time, the testmethod being characterized in that a boundary layer is provided betweenan electrode on at least one of the substrates and the liquid crystallayer, and judgment is made as to the presence or absence of aimage-sticking phenomenon after the liquid crystal optical element hasbeen left for 1 hour or more in a state of maintaining a predeterminedimage.

Further, according to a tenth aspect, there is provided a test methodfor a boundary layer of a liquid crystal optical element comprising apair of substrates with electrodes between which a liquid crystal layerincluding a chiral mematic liquid crystal is interposed wherein at leastone of the substrates is transparent; the liquid crystal layer exhibitstwo or more optically stable states in a non-voltage-application time,and a boundary layer is provided at least a part between an electrode onat least one of the substrates and liquid crystal, the test method for aboundary layer of a liquid crystal optical element being characterizedin that a test means having a predetermined ranking of hardness isprovided; the test means is brought to contact with the boundary layer Ntimes (1≦N) or more while a pressure is applied to the test means, andevaluation is made as to the surface hardness of the boundary layerbased on whether or not a flaw is resulted in the boundary layer,whereby applicability to the liquid crystal layer is determined.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic cross-sectional view showing an embodiment ofCL-LCD according to the present invention.

FIG. 2 is a diagram showing a pencil-scratching tester.

FIG. 3 is an explanatory diagram showing a pencil in contact with a testpiece.

FIG. 4( a) is an explanatory diagram showing a correct display of acharacter of “A”, and

FIG. 4( b) is an explanatory diagram showing a state of image-sticking.

FIG. 5( a) is a diagram showing a correct display of a character of “8”in a segment type CL-LCD, and

FIG. 5( b) is a diagram showing a state of image-sticking phenomenon.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is considered that the image-sticking phenomenon takes place by sucheffect that the orientation of liquid crystal molecules in a state thatthe previous phase state is preserved (hereinbelow, referred to as apreservation state) without applying no voltage to the liquid crystallayer, is fixed from a side of the interface. The interface is a regionbetween a boundary layer provided on an electrode and the liquid crystallayer. It is considered that the image-sticking phenomenon takes placeby that a state of the liquid crystal molecules at the interface isinfluenced with a lapse of time in a preservation state.

In the preservation state, there is a case that the orientation ofliquid crystal in an inner portion is changed so as to have a stableorientation because the orientation is fixed in a long time. When thesurface hardness of the boundary layer to which the liquid crystalcontacts is high (high hardness), the orientation of the liquid crystalmolecules at the interface changes inelastically. When the preservationstate is canceled and the orientation of the liquid crystal is changedto have a different state, a history remains in the orientation of theliquid crystal at the interface. Accordingly, the orientation of theliquid crystal in the preservation state may influence liquid crystal inthe vicinity of the interface.

On the contrary, in the boundary layer in contact with the liquidcrystal layer, when the heat distortion temperature is less than 50° C.;the coefficient of elasticity at room temperature is less than 1 kPa andthe boundary layer is very soft and has a high fluidity, the boundarylayer may be deformed with a change in a state of the orientation of theliquid crystal layer at a high temperature. Then, there causes a problemthat the boundary layer does not return to the original state, and thedisplay changes.

On the other hand, when the boundary layer in contact with the liquidcrystal layer has a low hardness and is relatively soft, liquid crystalmolecules in the vicinity of the interface can behave elastically.Namely, even though the orientation of the liquid crystal in thevicinity of the interface is able to be changed by suffering influencefrom the orientation of the liquid crystal in an inner portion to have astable alignment in the preservation state, the liquid crystal in thevicinity of the interface can return to the original state oforientation, as if an elastic deformation is restored, by applying avoltage to change the orientation of the liquid crystal in an innerportion. Accordingly, there is no possibility that the orientation ofthe liquid crystal in the vicinity of the interface in the preservationstate is fixed.

In other words, when the liquid crystal molecules are controlled byapplying a voltage from an outer side, and a display is performed byutilizing a memory state by canceling the voltage, it is preferable thatthe liquid crystal in an inner portion of the bulk of the liquid crystallayer and in the region of the interface can behave in substantiallysame manner. It is preferable that physical properties of the boundarylayer are determined in consideration of the liquid crystal used.

For this purpose, a material having a low elasticity and beingrelatively soft is used for the boundary layer in contact with theliquid crystal, whereby image-sticking in CL-LCD can be prevented.Further, as such boundary layer, a resin layer having a surface hardnessof “B” or less in a pencil hardness test method is used. As a polymermaterial used for the boundary layer, a material having durability toliquid crystal, temperature stability, easiness in manufacturing and asurface hardness having a predetermined softness may be used. It ispreferable that an aligning treatment by rubbing is not used to thesurface of the resin layer. The following test method can be employed tomeasure the surface hardness.

Test tools of Q kinds of hardness ranking each having different hardnessare prepared. Two or more test tools which are consecutive in hardnessranking are selected. They are brought to contact with the boundarylayer in contact with the liquid crystal layer N times (1≦N) or morerespectively while a pressure is applied, and a difference of flawsresulted in the boundary layer is observed. When a significantdifference is observed, surface hardness is determined. When the numberof times of the occurrence of flaws per N times is K times (1≦K≦N), theboundary layer satisfying the following conditions is found. Further,the test method may be simplified, and a predetermined standard ispreviously determined. Namely, judgment of 0/1 can be conducted in thedetection of once time. This method can be applied to a samplinginspection in a production process.

-   -   In a case of N=1 or 2,

-   (1) K=0 on the M th test tool and

-   (2) 1≦K on the M+1 th test tool    -   In a case of N=3 or more,

-   (3) K/N<0.4 on the M th test tool and

-   (4) 0.4≦K/N on the M+1 th test tool.

In the following, CL-LCD of the present invention will be described withreference to the drawings. FIG. 1 shows a cross-sectional view of anembodiment of CL-LCD. A first substrate 2 a is arranged at a front sideand a second substrate 2 b is arranged at a rear side wherein at least apart of the substrate 2 a is transparent. The substrate 2 b may beopaque. Glass or plastics may be used for the substrates. A blackcoating or a colored layer which absorbs or reflects partly visiblelight is arranged as a light absorbing layer 8 on an outer surface ofthe substrate 2 b at a rear side. Further, a color filter may beprovided at an inner surface side of the substrate to adjust visibilityand so on.

In the following, a case that the substrates 2 a, 2 b are bothtransparent will be described. A transparent electrode group 3 a isarranged at a front side and a transparent electrode group 3 b isarranged at a rear side so as to cross perpendicularly. Either isdetermined to be a row electrode group; the other is to be a columnelectrode group, and each crossing portion of the row electrode groupand the column electrode group is to be each pixel. The light absorbinglayer 8 is provided at the rear surface side so as to correspond to adisplay portion. Further, an electrical insulation layer 4 a is formedon the transparent electrode group 3 a, and a resin layer 5 a which isnot subjected to an alignment treatment by rubbing is formed on theelectrical insulation layer 4 a. The surface hardness of the resin layer5 a is determined to be “B” or less in a test value according to apencil hardness test which will be described later.

For example, the resin layer 5 a may be formed by using a polyimideresin. In this case, it is preferable that a state of the baked surfaceis used as it is. The resin layer may be formed on either one of thesubstrates 2 a, 2 b. However, it is preferable to form the resin layer 5b at a rear surface side in the same manner as at a display surfaceside.

The substrates 2 a, 2 b are pressed by interposing a peripheral sealingmaterial 6 to form a cell gap into which a chiral nematic liquid crystallayer 7 is introduced. The liquid crystal is driven by a voltage appliedacross the electrodes so that transition in a phase state is controlled.

Next, a test method for the boundary layer according to the presentinvention will be described. First, a test piece was prepared to testthe surface hardness, and tests were conducted with testing pencils anda pencil-scratching tester. The test method is based on in principle JISK 5400 8.4 (Japanese Industrial Standard defining the surface propertiesof coating or the like. There is ASTM D-3363, U.S.A. as a similar testmethod/standard). The method for forming a coating on the test piece andthe length on the test piece scratched with pencils were according todifferent procedures from JIS K 5400. The procedures are describedspecifically.

On a glass substrate having a size of 10 cm×10 cm and a thickness of 0.7mm, a solution of resin was coated by screen printing. It was baked at300° C. for 30 min to form a resin layer having a thickness of about 600Å. As the test piece, such one that the resin layer was formed, andthen, it was left one day or more, was used. The test piece formingprocedures are different from those of JIS K 5400.

Testing pencils as described in JIS S 6006 are used. As the testingpencils, there are those in a set having a series of density signsproposed by Japan Paint Inspection and testing Association. In thehardness of pencils, a density sign of 9H is the hardest and 6B is thesoftest. A harder one has an upper ranking. In the measurement, the coreof a testing pencil is exposed to about 3 mm in a circular column shapeby using a cutting knife. Then, the core is put on a polishing paperperpendicularly, and it is polished so that the top end is flat and theangle is acute. As the polishing paper, the polishing paper of No. 400described in JIS R 6252 is used.

As the test method, a scratching method by a human hand was not employedbut a method by a tester was employed. As the tester, a surface propertymeasuring instrument of TYPE-HEIDON: 14 (hereinbelow, referred to asmeasuring instrument) manufactured by Shinto Kagakusha was used. FIG. 2shows a state that a test piece and a pencil are attached to themeasuring instrument. In measuring the hardness of a resin layer, a testpiece 122 is attached to a test piece mounting table 123 of themeasuring instrument. In this case, the test piece is fixed horizontallywith an adhesive tape directing the coated surface upward.

The test piece 122 should be fixed to the test piece mounting table 123so as not to move in a direction of the movement of the table.Subsequently, a testing pencil 121 is selected, and it is attached to apencil holder 124 so that the top end of the core of the pencil isbrought to contact with the coated surface (the resin layer). FIG. 3shows a state that the core of the pencil having a flat top end and anacute edge is in contact with the coated surface at an angle of 45° C.

Then, a balance weight 125 is adjusted so that the load of the pencil121 applied to the test piece 122 is not deflected in a positive ornegative side. The pencil 121 is separated from the coated surface; alever 126 is fixed, and a weight 128 having 1.0 kg is placed on a weighttable 127 so that the pencil 121 is again brought to contact with thecoated surface, and the load of the weight 128 is applied to the top endof the core.

In this state, the pencil mounting table 123 is moved at a constantspeed of 0.5 mm/s. In the measuring instrument, the test piece mountingtable 123 is moved by driving a motor. By such movement, the coatedsurface of the test piece 122 is scratched in a length of about 10 mmwith the core of the pencil 122. Thus, a test of one time is finished.After the test of one time, the core of the pencil 121 is againpolished, and the same test is conducted. 5 Times of test are conductedto one pencil.

The procedures of measurement was principally based on JIS K 5400 asdescribed above provided that the length of scratching the test piecewith the pencils was determined to be about 10 mm. Although the lengthof scratching is 3 mm in JIS K 5400, the length was determined to belong in order to obtain easy confirmation of the presence or absence ofa flaw in the resin layer. Basically, the same effect is obtainable inthe case of 3 mm.

After 5 times of test were conducted on the one pencil, a pencil havingthe next density sign was selected to conduct 5 times of test similarly.These tests are conducted successively on pencils of different densitysign.

A pair of pencils whose density signs are adjacent to each other inwhich there is a pencil wherein a “drag mark” is recognized 2 times ormore among 5 times of test, and a pencil wherein the drag mark isrecognized less than 2 times, are selected. In the selected pair ofpencils, the density sign on the pencil wherein the occurrence of the“drag mark” is less than 2 times is determined to be the surfacehardness of the resin layer. Namely, a group of pencils of 17 grades:6B, 5B, 4B, 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H, 7H, 8H and 9H, isprepared. Then, the density sign of a pencil having the hardness at alower side, selected from this pencil group, wherein the occurrence ofdrag marks is less than 2 times among 5 times when a test piece isscratched each 5 times with two pencils having adjacent grades ofdensity sign, is adopted. Accordingly, in the pencil having a higherhardness ranking by 1 grade, the occurrence of the drag mark is 2 timesor more among 5 times.

Here, the “drag mark” means a flaw which slightly bites in the surfaceof the coated layer, and excludes a concave in the coated layer resultedfrom the application of a pressure. A distinguishable flaw under thefollowing conditions is defined as “drag mark”. Carbon powder is removedwith an erasing rubber so as not to damage the coated layer in thetested portions, and visual observation is conducted from a directionperpendicular to the direction of scratching and from an angle of 45° tothe test plane. As the erasing rubber, a plastic erasing rubberdescribed in JIS S 6050 is used.

Next, a method for manufacturing a liquid crystal optical element 1 willbe described. First, two substrates with transparent conductive layerssuch as ITO (indium tin oxide) were prepared. To each substrate, anelectrode group having predetermined interline spaces (spaces betweenelectrodes) and a predetermined number of electrodes were formed byetching, and an electrical insulation layer was formed at a side of thetransparent electrode forming surface of each of the substrates.

Then, a solution of polyimide resin was coated on the electricalinsulation layer of each of the substrates by a spin coating method; thesolvent was dried at 60° C., and it is baked at 300° C. for 30 min. Bysuch treatment, a resin layer having a layer thickness of about 600 Åwas formed.

Then, spacers of resin beads having a diameter of 4 μm were scattered onthe substrates, and a peripheral sealing material made of an epoxy resinincluding a slight amount of glass fibers having a diameter of 4 μm wascoated at 4 sides of the substrates excluding a portion as a liquidcrystal injection port. Then, the two substrates were bonded to therebyprepare a liquid crystal cell.

As a chiral nematic liquid crystal, 5.1 parts of chiral agent indicatedby chemical 1, 5.1 parts of chiral agent indicated by chemical 2 and 5.1parts of chiral agent indicated by chemical 3 were mixed to 84.7 partsof nematic liquid crystal having T_(c)=87° C., Δn=0.231 , Δε=16.5,viscosity η=32 mPa·s and relative resistivity=2×10¹¹Ω·cm. Thus preparedchiral nematic liquid crystal is referred to as liquid crystal P. Thehelical pitch of the liquid crystal P was about 0.34 μm.

After the liquid crystal P was injected to the liquid crystal cell by avacuum injection method, the injection port was sealed with aphotocurable resin. The injected liquid crystal P was directly on theresin layers. In this case, the surface hardness of the resin layers wasdetermined to be “B” or less in terms of pencil hardness. Further, ablack paint was coated on an outer surface of a rear side substrate inthe liquid crystal cell. Hereinbelow, Examples and Comparative Exampleswill be described.

EXAMPLE 1

CL-LCD was prepared according to the manufacturing method describedbefore. The line space was 10 μm and the number of stripe electrodes was4. Each resin layer as a boundary layer was formed by using a solutionof polyimide resin RN-1266 manufactured by Nissan Chemical Industries,Ltd. wherein the layer thickness was about 600 Å. In the measurement ofthe surface hardness of the resin layer according to thebefore-mentioned test method, it was found to be “B”. RN-1266 andRN-1286 which is used in Example 2 are a new material for an alignmentlayer, and the properties and so on are described in specificationsprovided by that company.

Then, a bipolar pulse of rectangular waveform (hereinbelow, referred toas a pulse A) having a pulse width of 900 ms and an effective value of20 Vrms was applied to all pixels through electrodes of both substrate.Then, liquid crystal P became FC, and all the pixels exhibited a uniformblack color. Then, a bipolar pulse of rectangular waveform (hereinbelow,referred to as a pulse B) having a pulse width of 100 ms and aneffective value of 40 Vrms was applied by selecting electrode lines ofthe both substrates. The liquid crystal P at pixels to which the voltagewas applied, became PL, and the pixels exhibited a selective reflectionof green color.

As described above, a state that a portion exhibiting a selectivereflection of green color and a portion exhibiting a black color weremixed in the same display surface, was provided. After the liquidcrystal optical element was put in a constant temperature of 60° C. for10 days, it was again taken out in an atmosphere of room temperature.The state of display at the time of taking out from the incubator wasthe same as that before putting into the incubator.

Next, the pulse A and the pulse B were alternately applied 3 times toall the pixels by selecting all electrode lines. When voltages wereapplied and stopped in the order of pulse A, pulse B, pulse A, eachpixels exhibited a uniform black color (FC) having no difference ofbrightness. Further, when voltages were applied and stopped in the orderof pulse B, pulse A, pulse B, each pixel exhibited a uniform selectivereflection of green color (PL). Thereafter, even when the application ofthe pulse A or the pulse B was repeated several times, a uniform greencolor or black color having no difference of brightness was exhibitedbetween pixels. As described above, in CL-LCD using the resin layershaving a pencil hardness “B” or less, the occurrence of image-stickingcould be suppressed.

EXAMPLE 2

A liquid crystal optical element was prepared by the same method as inExample 1 except that a solution of polyimide resin RN-1286 manufacturedby Nissan Chemical Industries, Ltd. was used. The line space was 10 μmand the number of stripe electrodes was 160. The surface hardness of theresin layers was “3B”.

First, initialization was performed by applying a relatively highvoltage to liquid crystal corresponding to each pixel to render theliquid crystal to be HO; by eliminating the voltage in such state, andapplying further a voltage which rendered the liquid crystal to be FC.After the initialization, a-line-at-a-time driving was performed in oneround with 160 row electrodes and 160 column electrodes to display acharacter of “A”, and then, the application of the voltage was stopped.As a result, a state that a character of “A” in a green color (PL) wasdisplayed in a background of black color (FC), was provided. After theliquid crystal panel displaying a character of “A” was preserved in aincubator of 60° C. for about 1000 hours, it was again taken out in anatmosphere of room temperature. The state of display at the time oftaking out from the incubator was the same as that before putting intothe incubator.

Next, the liquid crystal to all the pixels was made to be FC by the samedriving method as used for displaying a character of “A” before thepreservation whereby a black image was provided in the entire display.Further, the liquid crystal to all the pixels was made to be PL as well,whereby the entire display became a state exhibiting a selectivereflection of green color. Further, the liquid crystal to all the pixelswas again to be FC, whereby the entire display provides a black display,and then, the application of the voltage was stopped. In this case,there was no difference of brightness between pixels, and the entiredisplay exhibited a uniform black color. Thereafter, even when anattempt of providing a state of black display or a state of selectivereflection was made several times, a uniform image having no differenceof brightens could be obtained between pixels. In this Example too, theoccurrence of image-sticking could be inhibited by using resin layershaving a pencil hardness of “B” or less.

COMPARATIVE EXAMPLE A1

A liquid crystal optical element was prepared by the same method as inExample 1 except that a solution of polyimide resin SE-3840 manufacturedby Nissan Chemical Industries, Ltd. was used as resin layers. Thesurface hardness of the resin layers in this example was “3H”.

In the same manner as in Example 1, a state of that a portion exhibitinga selective reflection of green color and a portion exhibiting a blackcolor were mixed in the same panel, was provided. The panel was put in aincubator of 60° C. for 10 days. Then, it was again taken out in aatmosphere of room temperature. The state of display was the same asthat before putting into the incubator.

Next, the pulse A and the pulse B were alternately applied 4 times ormore to all the pixels by selecting the all electrodes. when the pulse Aand the pulse B were alternately applied in the order of pulse A, pulseB, pulse A, . . . , it was found that there was a difference ofbrightness between pixels and a uniform image could not be provided inthe entire display even in a case that the entire display was in a blackcolor, or a case that the entire display was in a green color (selectivereflection). The difference of brightness took place between pixelswhich displayed a black color and pixels providing the selectivereflection before the preservation in the incubator.

Even when voltages were applied in the order of pulse B, pulse A, pulseB, . . . after the panel was taken out from the incubator, the entiredisplay did not provide a uniform image. Further, after a lapse of 5days, there was no change in the state of display. Thus, image-stickingtook place in the case that the surface hardness of the resin layers incontact with the liquid crystal P was “3H”.

COMPARATIVE EXAMPLES A2, A3 and A4

Liquid crystal optical elements were prepared in the same manner as inExample 2 provided that the kind of a solution of resin used for formingresin layers was changed. The solution of resin by CHISSO CORPORATION isfor conventional TN or STN. In Comparative Example A4, no alignmentlayer was formed. Then, a character of “A” was displayed by the samedriving method as in Example 2, and then, the application of the voltagewas stopped. The liquid crystal in a background portion exhibited ablack color (FC), and the liquid crystal in a portion corresponding to acharacter of “A” exhibited a green color (PL). After the liquid crystalpanel was preserved in a incubator of 60° C. for about 1000 hours, itwas again taken out in an atmosphere of room temperature.

The entire display was made to be a black image by the same drivingmethod as used for displaying “A” before the preservation in the samemanner as Example 2. Further, the liquid crystal to the entire pixelswas made to be PL, and the entire display was to be a state of selectivereflection of green color. Further, the liquid crystal to the entirepixels was again to be FC, whereby the entire display provided a blackimage, and then, the application of the voltage was stopped.

Then, a degree of recognition of a image of “A” after a long timepreservation, namely, a degree of the occurrence of image-sticking wasexamined for each solution of resin used. Further, the surface hardnessof the resin layers by each of the solution of resin was measured by thebefore-mentioned method. Table 1 shows the surface hardness of the resinlayers and a degree of image-sticking in each Example. In Table 1, “◯”indicates a state that the character of “A” can not be observed at all,“X” indicates a state capable of being recognized, and “X×” indicates astate that it can be recognized strongly. Further, even after a lapse of5 days, there was no change in the degree of image-sticking in each ofthe liquid crystal panels.

TABLE 1 Degree of Kind of solution of resin Surface image- Example(Product code and producer) hardness sticking  1 RN-1266 Nissan ChemicalB ◯ Industries, Ltd.  2 RN-1286 Nissan Chemical 3B ◯ Industries, Ltd. A1SE-3840 Nissan Chemical 3H X Industries, Ltd. A2 A-2504 CHISSO 3H XCORPORATION A3 A-2710 CHISSO 3H X CORPORATION A4 (Nil) Only ITO >6H   XX

As shown in Table 1, image-sticking takes place when the surfacehardness of the resin layers is “3H” or the like. However, noimage-sticking takes place when the surface hardness is relatively lowas “B” or “3B”.

In the above-mentioned Examples 1 and 2, the surface hardness of theboundary layers were respectively “B” and “3B”. The resin layers havinga surface hardness within “B” to “3B” can be forced by adjusting arelative amount of the solution of resin used in each Example and mixingit.

Further, instead of polyimide, a predetermined compound may be selectedfrom a polymer material in consideration of the durability to liquidcrystal, the temperature properties long time stability, easiness informing the resin layers and so on.

In each of the above-mentioned Examples, a case that a resin layer isprovided between the entire surface of the transparent electrode and theliquid crystal layer is shown. Even in a case that the resin layer isprovided between a part of the transparent electrode and the liquidcrystal layer, image-sticking can be prevented in a range where theresin layer is provided.

Since it is considered that the image-sticking takes place by that theorientation of liquid crystal in a state of preservation affects liquidcrystal molecules fixed at the interface, it is preferable that theresin layer having a lower hardness is directly on the liquid crystal.

The liquid crystal optical element shown in each of the above-mentionedExamples has such structure that two substrates have respectively aplurality of electrodes. However, a structure that either of thesubstrates has an electrode which is divided into a plurality ofportions and the other has a non-divided common electrode (a sheet-likeelectrode) may be used.

In the test method of the present invention, a state of standing meansthat a state of display is maintained as it is in a predeterminedatmosphere without applying a driving voltage. In this case, inspectionfor image-sticking can be conducted in a shorter time than thebefore-mentioned Examples by adjusting external circumstances such astemperature. For example, in conducting sampling inspection for each lotor a periodical inspection of evaluating process in mass production, aresult can be obtained preferably in a short cycle.

Further, the present invention is applicable not only to a liquidcrystal display element for performing dot matrix driving but also aliquid crystal optical element for performing segment display type andstatic driving. Further, in the liquid crystal optical element in thepresent invention, the liquid crystal may be a cholesteric liquidcrystal.

In accordance with the present invention, the occurrence ofimage-sticking can be prevented by providing a resin layer having asurface hardness of B or less in a pencil hardness test method, betweena part of or the entire surface of the electrode of at least onesubstrate and the liquid crystal.

Further, the liquid crystal optical element of the present invention canbe used for a public information display apparatus, a display device fora portable equipment, a display device for an equipment of measurement,a display device for a private electronic equipment and so on. Further,a display of high quality having a memory effect can be provided.

1. A liquid crystal optical element, comprising: a pair of substrateswith electrodes between which a liquid crystal layer comprising a chiralnematic liquid crystal is interposed; wherein at least one of thesubstrates is transparent; wherein the liquid crystal layer exhibits twoor more optically stable states containing a planar state and a focalconic state in a non-voltage-application time; wherein the liquidcrystal optical element, which produces a selective reflection,comprises a boundary layer comprising polyimide which is providedbetween an electrode on at least one of the substrates and the liquidcrystal layer; and wherein the surface hardness of the boundary layer isB or less in terms of a pencil hardness test.
 2. The liquid crystaloptical element according to claim 1, wherein the electrode on at leastone of the substrates is divided into a plurality of portions.
 3. Theliquid crystal optical element according to claim 1, wherein the surfacehardness is 3 B or less.
 4. The liquid crystal optical element accordingto claim 1, wherein a dot matrix display is performed with theelectrodes.
 5. The liquid crystal optical element according to claim 1,wherein a segment display is performed with the electrodes.
 6. A testmethod for a liquid crystal optical element, comprising: judging thepresence or absence of a image-sticking phenomenon after the liquidcrystal optical element has been left for 1 hour or more in a state ofmaintaining a predetermined image; wherein said liquid crystal opticalelement is as claimed in claim
 1. 7. A test method for a boundary layerof a liquid crystal optical element, comprising: providing a test meanshaving a predetermined ranking of hardness; contacting the test meanswith a boundary layer of said liquid crystal optical element N times(1≦N) or more while a pressure is applied to the test means evaluatingthe surface hardness of the boundary layer based on whether or not aflaw has resulted in the boundary layer, thereby determiningapplicability to a liquid crystal layer of said liquid crystal opticalelement; wherein said liquid crystal optical element is as claimed inclaim
 1. 8. A public information display apparatus, comprising theliquid crystal optical element as claimed in claim 1.